The present invention is concerned with the steering of a vehicle having an electrically assisted steering system (EAS) when running in the situation of ABS split mu operation, where the nearside and offside wheels of the vehicle are running respectively on relatively high mu and relatively low mu surfaces, or vice versa resulting in the necessity for asymmetric brake force maneuvers.
Electric assist steering systems are well known in the art. Electric assist steering systems that use, for example, a rack and pinion gear set to couple the steering column to the steered axle, provide power assist by using an electric motor to either apply rotary force to a steering shaft connected to a pinion gear, or apply linear force to a steering member having rack teeth thereon. The electric motor in such systems is typically controlled in response to (a) driver's applied torque to the steering wheel, and (b) sensed vehicle speed.
Other known electric assist steering systems include electro-hydraulic systems in which the power assist is provided by hydraulic means under at least partial control of an electronic control system.
In the latter conditions, where a split mu braking operation is taking place, the unbalanced braking torques which occur can adversely affect the vehicle stability and tend to cause the vehicle to spin.